beta-Glucosaminidase activity of soils: effect of cropping systems and its relationship to nitrogen mineralization

The enzyme beta-glucosaminidase (EC 3.2.1.30) is involved in C and N cycling in soils. The effects of crop rotations and N fertilization on beta-glucosaminidase activity and its relationship to N mineralization were studied in soils of two long-term field experiments involving different cropping systems at the Northeast Research Center (NERC) and Clarion-Webster Research Center (CWRC) in Iowa that were initiated in 1978 and 1954, respectively. Surface soil samples (0-15 cm) were taken in 1996 and 1997 in corn (Zea mays L.), soybean [Glycine max (L) Merr.], oats (Avena sativa L.), or meadow (alfalfa; Medicago sativa L.) plots that received 0 or 180 kg N ha-1 before corn and an annual application of 20 kg P and 56 kg K ha-1. The beta-glucosaminidase activity in the soils was assayed at optimal pH (acetate buffer, pH 5.5); microbial biomass C (Cmic) and N (Nmic) were determined by chloroform-fumigation methods; N mineralization was studied in leaching columns under aerobic conditions at 30 degrees C for 24 weeks. The activities of beta-glucosaminidase were significantly affected by crop rotation (P <0.001) and N fertilization (P ranging from 0.05 to 0.001). Generally, the highest enzyme activity was obtained in soils under 4-year corn-oats-meadow rotations taken under meadow, and the lowest under continuous mono-cropping systems. The activity of this enzyme was significantly correlated with Corg (r ranging from 0.42** to 0.76***), Norg (ranged from not significant at one site one year to r = 0.76***), Cmic (r ranging from 0.44** to 0.71***), and Nmic (r ranging from 0.33* to 0.76***) in soils, and with cumulative N mineralized (r >0.84*** and r >0.79*** at the NERC and CWRC sites, respectively). The results suggest that beta-glucosaminidase plays a major role in N mineralization in soils and is affected by cropping systems, i.e. ecosystem function and health.